Anal Sex is stimulation of the anus during sexual activity. It can be done in several different ways: manually, orally or by anal intercourse. Anal sex can be the primary form of sexual activity or it can accompany other types of stimulation. For instance, couples sometimes include manual stimulation of the anus (either lightly rubbing the rim or inserting a finger into the anus) during vaginal intercourse. Others use the tongue in a similar fashion for oral stimulation. Anal intercourse is the insertion of a man's penis into his partner's rectum. Although anal sex is often thought of as a strictly homosexual activity, many heterosexual couples enjoy it too.
Anal penetration can be pleasurable, but it can also be a source of physical discomfort. The muscle on the outside of the rectum, called the anal sphincter, ordinarily tightens if stimulated, which means that attempts at insertion of the penis, or even a finger, may be uncomfortable even if done slowly and gently. If penetration into the anus is forced, injury is possible. It is helpful to use a lubricant liberally and to relax and gradually dilate the sphincter by gentle manual stimulation before attempting penetration.

Some people clear the rectum with a small disposable enema before anal intercourse. Because the rectum contains infection causing bacteria, anything (fingers, objects, penis) that has had contact with the anus should not subsequently be in contact with the vagina or mouth until it has been thoroughly washed.

Some people have strong negative attitudes toward anal sex, whether it takes place between homosexual or heterosexual couples. They may think of it as being unclean, unnatural, perverted, disgusting, or simply unappealing. It is sometimes regarded as the ultimate in depravity and has regularly been condemned by religious and secular authorities. Historically, religious objections originate from the fact that a woman cannot possibly get pregnant through anal sex, as some religions only approved of sexual intercourse for the purpose of reproduction.

Anal sex is subject to legal restrictions in some states, even between married couples. In general these statutes refer to anal sex acts as "crimes against nature", going back to the view that heterosexual intercourse, with its reproductive potential, is the only natural, healthy, non-sinful way of having sexual relations. These laws are strongly biased toward the prosecution of homosexuals because penetrative anal sex is far more common among gay men than it is among heterosexual couples.

Despite religious and legal prohibitions to anal sex, many people, regardless of their sexual orientation, have engaged in anal sex and found it pleasurable. They regard it as a legitimate form of sexual expression and as one of the fulfilling ways in which people can express their desire and affection for each other. It is up to each couple to decide what is acceptable and enjoyable for them. If a couple's attitudes or values make anal sex unacceptable, or if they do not enjoy it, they should feel no pressure to engage in it.

It is important to note that HIV, the virus causing AIDS, can be transmitted through anal sex, especially anal intercourse. In fact, the risk of HIV transmission is greater than it is in vaginal intercourse because the lining of the rectum tears more easily than the vagina does. The resulting skin breaks and bleeding increase the possibility of the transmission of bodily fluids containing the virus that causes AIDS. For those who choose to engage in either vaginal or anal penetrative sexual acts, using latex condoms is the best means of reducing the likelihood of HIV transmission.